Among the widely used electrochemical sensors with a sensor element and/or sensor housing consisting of glass, so-called non-glass sensors are gaining in importance. An application for non-glass sensors is for example the measurement of concentrations in production processes in the food industry. In particular the measurement of pH values that were previously measured primarily with glass electrodes is or will become outlawed in many industrial situations. The purpose of the new legislation is to avoid the dangers associated with a possible breakage of glass parts, e.g., in a production chain in the food industry.
The design of a glass-free electrochemical sensor poses on the one hand a problem that instead of a sensor housing made of glass, an enclosure of a different material is to be selected. As a solution, there are a variety of suitable polymer materials available. The material should meet application-specific requirements, i.e., adequate mechanical stability and chemical resistance, and it should be a good electrical insulator. On the other hand, there is a problem of finding a glass-free alternative for the sensor membrane, in particular for pH-sensors which usually contain a membrane of pH glass as a essential component of the measuring system. This makes it appropriate to use a different measuring principle.
To provide glass-free ion-sensitive elements that can in particular be configured as glass-free pH-sensitive elements, ion-sensitive field effect transistors, hereinafter referred to by the customary acronym ISFET, present themselves as a solution. For example, a method and a circuit for measuring ion activity in liquids involving the use of an ISFET are described in DE 3116884 A1. The ISFET in this case contains an insulating layer acting as transistor gate. The insulating layer, which has the function of an ion-sensitive surface area, is arranged between a source electrode and a drain electrode. A pH-measuring device with an electrode including an ISFET is described, e.g., in EP 63213 B1.
The aforementioned documents give only a schematic indication of the arrangement of the ISFET in the sensor. However, when attempting to implement this concept in practice, it becomes evident that the positioning, mounting and the electrical connections of the ISFET are of importance for a functional sensor. As a ground rule, it should be ensured that certain parts of the sensor are sealed tightly against leaks and moreover that the ISFET is protected against excessive mechanical stress, e.g., in applications involving increased pressure.
Sensors which belong generically to the same type are presented in U.S. Pat. No. 6,117,292 and U.S. Pat. No. 6,153,070. They have a sensor housing containing an ISFET in the shape of a small disc. A source connection and a drain connection are arranged on the rear side of the disc. At the front side, the disc has an ion-sensitive surface area for immersion into a measuring medium. The sensors have a stacked configuration in which the ISFET is clamped against a plate-shaped base that serves as a mechanical seat and at the same time as a lead-off element. The latter function is achieved through a design with direction-specific electrical conductivity due to a laminated structure of the plate-shaped base with alternating insulating- and conducting layers running substantially perpendicular to the main plane of the plate-shaped base. As a result, the base is electrically conductive in the direction perpendicular to its main plane and at the same time electrically insulating in one direction of the main plane. This arrangement provides the electrical connections to the source- and drain contacts on the rear side of the ISFET. Drawbacks of these sensors lie in their complexity and high manufacturing cost. In addition, the desire to arrange the source- and drain connections on the rear side of the ISFET represents a particular disadvantage, because it does not conform to the customary design of ISFETs and thus adds to the cost and limits the selection of available types of ISFETs.
A reference electrode is described in NL 1003458 C, which instead of the customary Ag/AgCl half cell with glass frit and conductive gel uses an Ag platelet coated with an AgCl film, with a thin SiO2 film arranged on top of the AgCl film. A clamping element serves to clamp the Ag platelet against an end surface of a sensor housing, so that the rear surface of the platelet, which faces away from the AgCl-coated side bears against the end surface of the sensor housing. The clamping element has a central opening leaving a mid-portion of the AgCl film exposed. A substantially ring-shaped sealing element is arranged between the clamping element and the AgCl platelet, surrounding the mid-portion of the AgCl as a tight seal against leakage of the measuring medium. It is mentioned in NL 1003458 C that the clamping element (referred to in Dutch as “kopdeel”, meaning “head part”) is attached to the sensor housing (referred to as “huls”, meaning “casing”) by means of an adhesive bond, but preferably by ultrasonic welding. However, this reference gives no indication regarding the spatial arrangement of the ultrasonic welding bond and in particular whether it forms a leak-tight seal running in a closed circle. There is furthermore no indication on how to perform the welding procedure. The ultrasonic welding process as well as other kinds of vibratory welding present a problem that in addition to the components to be bonded together, adjacent components are also subjected to some degree of vibration. This may be tolerable in the case of the Ag platelet of the aforementioned reference. However, if an ISFET were to be installed instead of the Ag platelet, the vibrations reaching the ISFET would cause cracks or separation of the film layer. Furthermore, the wire bonds of the lead-off wires to the source- and drain contacts of some types of ISFETs are particularly prone to getting damaged by vibrations. These kinds of problems do not exist in the reference electrode described in NL 1003458 C and accordingly this reference offers no suggestion whatsoever for a solution. Furthermore, the arrangement described in NL 1003458 C is designed with electrical contacts arranged on the rear side, analogous to the arrangements of U.S. Pat. No. 6,117,292 and U.S. Pat. No. 6,153,070, and is therefore not suitable for the particularly common type of ISFETs with electrical leads on the front side.